let x = 1;

function f(y = x) { 
  // 等同于 let y = x  
  let x = 2; 
  console.log(y);
}

f(7) // 1
console.log(Object.keys(['a','b']))
for (let index of ['a', 'b'].values()) {
    console.log(index);
  }
  let str = "352255";
// let unique = [...new Set(str)].join(""); // "352"
unique = new Set(str)
console.log(unique)
